Isotope selective ionization of tellurium dimers Te2 formed by excimer laser photodissociation of an organometallic compound: CH3TeTeCH3

Usingone standard commercial dye laser, tellurium dimers Te₂ — formed by excimer laser induced photoelimination from CH₃TeTeCH₃ at room temperature — are ionized isotope selectively.     

Photochemical effects of KrF excimer excitation in laser-induced fluorescence measurements of OH in combustion environments

This paper presents experimental evidence that using the KrF excimer laser for quantitative laser-induced fluorescence (LIF) studies of the OH A-X (3,0) system is highly problematic if the effects of both photobleaching and photochemistry are not included for laser spectral irradiances greater than 20 MW/cm² cm^-1. Pump-probe and time-resolved measurements of the OH LIF signal in an atmospheric pressure, premixed CH₄-air flame at low- and high-laser-spectral-irradiance conditions show that a significant amount of OH is produced from photofragments resulting from the simultaneous 2-photon predissociation of H₂O molecules in the C-X system. A 5+2-level rate-equation model that includes the effects of both photobleaching and photochemical OH production is shown to satisfactorily predict the data using a single adjustable parameter given by the effective, spectrally integrated 2-photon cross-section of H₂O near 248 nm. The time-integrated OH LIF signal was found to depend on both the laser spectral irradiance and the local concentration of H₂O. Additionally, use of the KrF excimer laser for 2-line rotational thermometry can produce temperature errors as great as +550 K at high laser-pulse energies. Received: 21 August 2000 / Revised version: 30 October 2000 / Published online: 21 February 2001     

Influence of laser fluence and irradiation timing of F2 laser on ablation properties of fused silica in F2-KrF excimer laser multi-wavelength excitation process

A collinear irradiation system of F₂ and KrF excimer lasers for high-quality and high-efficiency ablation of hard materials by the F₂ and KrF excimer lasers’ multi-wavelength excitation process has been developed. This system achieves well-defined micropatterning of fused silica with little thermal influence and little debris deposition. In addition, the dependence of ablation rate on various conditions such as laser fluence, irradiation timing of each laser beam, and pulse number is examined to investigate the role of the F₂ laser in this process. The multi-wavelength excitation effect is strongly affected by the irradiation timing, and an extremely high ablation rate of over 30 nm/pulse is obtained between -10 ns and 10 ns of the delay time of F₂ laser irradiation. The KrF excimer laser ablation threshold decreases and its effective absorption coefficient increases with increasing F₂ laser fluence. Moreover, the ablation rate shows a linear increase with the logarithm of KrF excimer laser fluence when the F₂ laser is simultaneously irradiated, while single KrF excimer laser ablation shows a nonlinear increase. The ablation mechanism is discussed based on these results. Received: 16 July 2001 / Accepted: 27 July 2001 / Published online: 2 October 2001     

Femtosecond uv excimer laser ablation

Experiments on the ablation of polymethylmethacrylate (PMMA) with 300 fs uv excimer laser pulses at 248 nm are reported for the first time. With these ultrashort pulses, ablation can be done at fluences up to five times lower than the threshold fluence for 16 ns ablation of PMMA, and the surface morphology is improved, also for several other materials. A model for ablation is proposed, assuming a non-constant absorption coefficient _eff depending on the degree of incubation of the irradiated material and the intensity of the incoming excimer laser pulse. The agreement between our model and our experimental observations is excellent for 16 ns excimer laser pulses, also predicting perfectly the shape of a pulse transmitted through a thin PMMA sample under high fluence irradiation. Qualitative agreement for 300 fs excimer laser pulses is obtained so far.     

Deep etching of epitaxial gallium nitride film by multiwavelength excitation process using F2 and KrF excimer lasers

The deep etching of GaN(0001) thin films epitaxially grown on Al₂O₃(0001) has been investigated by laser ablation using F₂ and KrF excimer lasers. The simultaneous irradiation with F₂ and KrF excimer lasers markedly improves etching quality compared with single-KrF excimer laser ablation and provides almost the same quality as that in the case of single-F₂ laser ablation with a high etching rate. Additionally, the present method achieves the deep etching of a GaN film of more than 5 μm in depth with steep side walls at an angle of 87° by changing the laser incidence angle. PACS 52.38.Mf; 61.82.Fk; 81.65.Cf     

XUV generation from plasma produced by a XeCl excimer laser on a Cu target

Summary We present a detailed study of XUV and soft X-ray emission from Cu plasma produced by an excimer laser at intensitiesI _L≦8·10¹¹ W/cm². The XeCl excimer laser (ψ≈308 nm) delivers pulses with energyE _L≈2.3 J, temporal durationt _L≈100 ns and brightnessB≧10¹⁴ W/cm² sr. We recorded a spectral conversion efficiency η=0.5% eV⁻¹ forI _L=4·10¹¹W/cm² in the aluminium window at 73eV with a harder X-ray tail around ≈400eV. We also measured the dependence of X-ray signal on laser intensity and viewing angle. Experimental results have been compared with some analytical laser-plasma interaction models.     

Comparison of electron beam and pulsed laser irradiation effects in Fe80B20

The effects of high-energy electron beam and pulsed excimer laser irradiation on the magnetic properties of Fe₈₀B₂₀ amorphous alloy are investigated by means of transmission and conversion electron Mössbauer spectroscopy (CEMS) and scanning electron microscopy (SEM). Both types of irradiation were found to induce changes in the magnetic anisotropy of Fe₈₀B₂₀, but the influence of high-energy electron beam irradiation was weaker. Depending on the number of applied laser pulses and repetition rate, controlled magnetic anisotropy and/or relaxation of internal stresses could be obtained by excimer laser irradiation. The presence of molten zones subsequently solidified and of laser-induced internal stresses supports the model of closúre domain structure.     

Reaction kinetics of excimer lasers using NF3

Disappearance of NF₃ and appearance of NF₂ and N₂F₂ in an excimer laser is followed quantitatively with a mass spectrometer. A model for the kinetics is presented.     

Excimer ArF laser with an output energy of 1.3 J at 2.0% efficiency on the He:Ar:F2 mixture

In this paper it is shown that to achieve a maximum efficiency and high output energy of an ArF (193 nm) excimer laser, one should use optimal pump intensity. It has been shown experimentally that the optimal pump intensity for an ArF excimer laser with the mixture of He:Ar:F₂ has a value of 4.5–5.0 MW/cm³. The results of an experimental study of the pump and active medium parameters effect on the efficiency and output energy of the ArF excimer laser on the mixture of He:Ar:F₂ are presented. To provide high pump intensity of an active medium, the excitation scheme of the LC-inverter type has been used where the current return conductor inductance had been increased from 30 to 80 nH. This allows the pump to achieve levels of intensity above 5.0 MW/cm³. By using the pump intensity of 5.0 MW/cm³ in an active medium of He:Ar:F₂–79.7:20:0.3 at total pressure of 2.4 atm, we are the first to obtain the output energy of 1.3 J at the total efficiency of 2.0%. The pulse duration (FWHM) was 15±1 ns and the peak pulse power was 85 MW. PACS 42.55.Lt; 42.60.Lh     

Effect of excimer laser annealing on the properties of ZnO thin film prepared by sol-gel method

Pristine ZnO thin films have been deposited with zinc acetate [Zn(CH₃COO)₂], mono-ethanolamine (stabilizer), and isopropanol solutions by sol-gel method. After deposition, pristine ZnO thin films have been irradiated by excimer laser (λ = 248, KrF) source with energy density of 50 mJ/cm² for 30 sec. The effect of excimer laser annealing on the optical and structural properties of ZnO thin films are investigated by photoluminescence and field emission scanning electron microscope. As-grown ZnO thin films show a huge peak of visible region and a wide full width at half maximum (FWHM) of UV region due to low quality with amorphous ZnO thin films. After KrF excimer laser annealing, ZnO thin films show intense near-band-edge (NBE) emission and weak deep-level emission. The optically improved pristine ZnO thin films have demonstrated that excimer laser annealing is novel treatment process at room temperature.     

Deposition of refractory coatings by LCVD

TiC layers have been successfully deposited locally on tool steel specimens and real small industrial tools in order to improve their mechanical and corrosion properties. TiC films have been produced by the pyrolytic laser chemical vapour deposition technique, using a low power RF-modulated CO₂ laser and a gas mixture of TiCl₄/CCl₄/H₂/H/Ar. The surface temperature was monitored by a microcomputer-based feedback loop system that was specially developed and integrated to control the laser. SEM, EDX, XRD and TEM analysis showed that close stoichiometric TiC (fcc structure, lattice constant a=4.2Å) was deposited in the films with nanocrystalline structure. The microhardness of the coatings was in the range 2500–3200HK. Furthermore, an ArF excimer laser was used to enhance the deposition rate.     

Stimulated emission of argon dimers excited by a pulsed plasma-cathode discharge

Results are reported from a theoretical and experimental investigation aimed at obtaining lasing on transitions of an Ar₂ excimer laser pumped by a pulsed electric discharge. It is shown that by using a recently developed plasma-cathode discharge, quasi-cw excitation of the laser active medium can be achieved with a gain of around 0.1 cm⁻¹. The results of preliminary experiments performed using a model system indicate an effect wherein the duration of the spontaneous emission at the gl=126 nm transition of the Ar₂ dimer decreases monotonically as its peak intensity increases. This behavior, which is observed with increasing pump intensity, is evidence of a stimulated emission effect in the system. However, the measurements indicate that no amplification takes place in the active medium because of accidental impurities in the working gas. Zh. Tekh. Fiz. 67, 49–52 (November 1997)     

Boron doping of silicon by excimer laser irradiation in a reactive atmosphere

UV excimer lasers have been used to dope semiconductors by a one-step process in which the laser serves both to melt a controlled thickness of a sample placed in dopant ambient and to photodissociate the dopant molecules themselves. Here we report the boron doping of silicon by means of an ArF (193 nm) excimer laser. Dopant atoms are obtained by photolysis of BCl₃ or pyrolysis of BF₃ molecules. The doping is performed both in gas ambient and using only an adsorbed layer. We have investigated the dependence of doping parameters such as laser pulse repetition and gas pressure on the subsequent boron impurity profiles and the dopant incorporation rate. These results indicate that the laser doping process is dopant-flux limited for BF₃ and externally rate limited for BCl₃.     

Plume temperature in the laser ablation of polyimide films measured by infrared emission spectroscopy

The infrared emission spectrum of the plume produced by KrF excimer laser ablation of polyimide films in air and in He was measured in the 680 to 1580 cm^–1 wavenumber range. Using 400 mJ/cm² laser pulses of 248 nm and 35 ns duration yielded a strong emission band characteristic of thev ₂ transitions of hot HCN molecules. Band counters calculations were carried out of thev ₂ emission expected from HCN in thermal equilibrium at various temperatures. They indicate that except for a slight deviation of the measured data from thermal equilibrium, the best fit of the observed results is obtained at a plume temperature of 2250±150K.     

Dose and pressure dependence of silicon microstructure in SF6 gas due to excimer laser irradiation

ArF excimer laser radiation on Si surface at controlled SF₆ atmosphere creates a microstructure whose alteration at various UV doses and SF₆ pressures are investigated in this work. Moreover, a rigorous model has been presented here regarding the experiments based on the micro-fluid mechanical properties of molten silicon layer and the subsequent mechanical wave resonance due to the surface shallow fluid theory. The competitive thermal and non-thermal UV laser interaction with Si and the following heat transfer explains the creation of the self-assembled micro-pillars on Si surface.     

Excimer laser-induced transformation in laser ablated Pb(Zr0.52Ti0.48)O3 amorphous thin films

Pb(Zr_0.52Ti_0.48)O₃ (PZT) amorphous thin films were deposited on Si substrates at room temperature and 573?K by pulsed laser deposition. The as-deposited films were subsequently annealed at various laser power densities using a KrF pulsed excimer laser irradiation to induce the phase transformation from amorphous to ferroelectric perovskite structure. Structural analysis shows the possibility of transformation from pyrochlore to perovskite transformation when irradiated above a laser power density of 1.4?MW/cm², which is in agreement with the thermal simulation. The surface quality of the PZT films deposited on 573?K is remarkably superior to that deposited at room temperature due to the enhanced thin structure and composition homogeneity. Almost all the pyrochlore phase transformed into perovskite structure after annealing at 2.8?MW/cm² for 120?s for both PZT films deposited at room temperature and 573?K, respectively. P-E hysteresis measurement of the laser-treated PZT shows relatively low remnant polarization P _r of about 1.2?μC/cm².     

Studies on excimer laser doping of GaAs using sulphur adsorbate as dopant source

Excimer laser doping of GaAs using sulphur adsorbate as a dopant source is demonstrated. Box-like n-type layers of depths of about 100 nm with carrier concentration as high as (23)×10¹⁹ cm^–3 are formed. Passivation of GaAs using a (NH₄)₂S_x solution for 40 min followed by sublimation of the excess sulphur atoms in high vacuum result in an effective dopant for controllable n-type doping. The samples are irradiated using a KrF excimer laser in a N₂ gaseous environment. Secondary ion mass spectrometry (SIMS) measurements show that sulphur is successfully incorporated in the GaAs. The sheet resistance is controlled by adjusting the laser energy fluence and number of laser pulses. Rutherford backscattering spectrometry with channeling (RBS/C) alignment measurement indicates that lattice damage is undetectable for N₂ gas pressures of 760 Torr.     

V centers produced in KI crystals under KrCl excimer laser irradiation

Abstract

Irradiation of KI near 150 K with KrCl excimer laser irradiation (hv=5. 58 eV) produces V centers causing V₂ and V₃ bands. The two bands exhibit 100-type dichroism. In KI containing V centers, the 111 cm^?1 Raman signal attributed to I_3- molecular ions is observed. Under KrCl excimer laser irradiation at low temperatures, resonance Raman scattering effects have been also studied for KI, NaI and LiI.